PET is a relatively advanced clinical imaging technique in the field of nuclear medicine. It plays an important role in medical research and clinical diagnosis.
PET is a diagnostic technique that is used to observe functions and metabolism of human organs and tissues at the molecular level. For PET, a positron radioactive drug (e.g., 18F-FDG) will be injected into a human body. Because the metabolism of fludeoxyglucose (FDG) is similar to glucose, the FDG will gather in cells that digest the glucose. The uptake of the radioactive drug by rapidly growing tumor tissues is different. A positron emitted by the decay of 18F and an electron in tissues will undergo an annihilation reaction to generate two γ photons with the same energy in opposite directions. A detector array surrounding the human body will detect the two photons using a coincidence measurement technique, and determine position information of the positron. A tomography image of positrons in the human body can then be constructed by processing the position information using an image reconstruction software.
FIG. 1 is a conventional structure of a PET system. As shown in FIG. 1, a gantry 11 includes a channel 12. An examination table 13 can move into or out of the channel 12. A patient to be examined can lie on a table surface 14 of the examination table 13. When the examination table 13 moves into the channel 12, a detector array around the channel 12 arranged in the gantry 11 can detect photons generated during the reaction.
A length of an axial field of view (FOV) of the present PET system is limited to about 25 centimeters that is much shorter than the length of a human body, e.g., 2 meters. In order to improve the PET system, there has been a plurality of PET detector units set along the axial direction of the gantry to expand the FOV in the axial direction of a body. For example, when the FOV in the axial direction of the body is expanded to about 2 meters, the whole body from the head to toes of the patient can be diagnosed simultaneously.
Expanding the FOV may enhance a sensitivity by dozens of times, but this may lead to a high counting rate that cannot be processed by a current electronic logic. Meanwhile, the speed of image reconstruction using a traditional PET technique is slowed down by dozens of times because of the high counting rate.